Token passing protocol for RFID systems

ABSTRACT

Methods, systems and apparatuses for RFID readers forming a reader network are described. In an aspect of the present invention, a plurality of RFID readers are configured to interrogate tags. Furthermore, the readers are configured to communicate with one another by transferring a token, represented by a signal. Possession of the token enables the reader to access a RF communications medium. Readers can be arranged in a ring configuration, and interconnected via wired links. A secondary token may circulate in the ring in addition to the primary token, to ensure redundancy in the system. A reader waits for a predetermined time interval before accessing the RF communications medium.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to radio frequencyidentification (RFID) systems, and more particularly to systems andmethods for communications among networked RFID readers to achieveefficient tag interrogations.

2. Background Art

Radio frequency identification (RFID) tags are electronic devices thatmay be affixed to items whose presence is to be detected and/ormonitored.

The presence of an RFID tag, and therefore the presence of the item towhich the tag is affixed, may be checked and monitored by devices knownas “readers.” Readers typically have one or more antennas transmittingradio frequency signals to which tags respond. Once a reader receivessignals back from the tags, the reader passes that information indigital form to a host computer, which decodes and processes theinformation.

With the maturation of RFID technology, efficient communication betweentags and readers has become a key enabler in supply chain managementespecially in manufacturing, shipping, and retail industries, as well asin building security installations, healthcare facilities, libraries,airports etc.

In some environments, multiple readers may be present. It may beadvantageous for a particular group of RFID tags to be interrogated bymore than one reader. Various RFID communication protocols enable thisfunctionality. For example, the emerging standardized RFID communicationprotocol known as Gen 2, allows for RFID tags to be commanded into anumber of possible “states,” allowing several readers to interrogate thesame tag population.

However, when multiple readers simultaneously send out interrogationsignals to an overlapping group of tags, this may cause interferencebetween the interrogation signals. As a result, there may be signalconflict and/or signal degradation, resulting in loss of information.The signal degradation is especially prominent when readers are confinedto a fixed frequency or a narrow band of frequency, and have limitednumber of allowed communication channels.

Thus, what is needed are more efficient and reliable ways for multipleRFID readers to efficiently communicate with RFID tags without unwantedinterference.

BRIEF SUMMARY OF THE INVENTION

Methods, systems, and devices for operation of RFID readers in anetworked configuration are described. The networked readers circulate atoken among themselves to efficiently time-multiplex access to acommunications medium.

In aspects of the present invention, readers can be arranged in a ringconfiguration, and interconnected via wired links transmitting tokensignals. A first reader can “possess” the time interval that it requiresto interrogate RFID tags in its read zone. When the first reader hascompleted an interrogation, it can relinquish access to the RF medium bysignaling a subsequent reader, i.e. the first reader passes the token tothe subsequent reader. After performing an interrogation, the subsequentreader can pass the token to still another reader. Eventually, the tokensignal completes a circuit around the ring of readers, and the firstreader again possesses access to the RF medium. This process may berepeated as often as desired.

In an example aspect of the present invention, a secondary token maycirculate in the ring in addition to the primary token, to provideredundancy in the system. The reader that receives the primary token isallowed to access the RF communications medium immediately, while thereader that receives the secondary token thereby becomes aware that itwill possess the primary token next. A secondary token may help inefficient power management and fast operation in a network.

Aspects of the present invention include waiting for a predeterminedtime interval before accessing the RF communications medium. A length ofthe predetermined time interval depends on the system environment. Areader is configured to generate a token to keep ring integrity intact,maintaining the process of sequential, time-multiplexed taginterrogation.

In other example aspects of the invention, readers are capable ofretaining the token for a variable period of time to allow a second RFsystem to perform a RF interrogation utilizing the same RF spectrum,and/or to allow readers with higher priority to possess the RF mediumfor an extended period of time.

Aspects of the present invention include readers arranged in amaster-slave configuration, where a master reader can pass a slave tokento one or more slave readers. Yet another aspect of the inventionincludes readers arranged in spatially separated local networks. Groupsof readers may operate synchronously even if they belong to spatiallyseparated local loops to mitigate unwanted interference.

In another aspect of the present invention, a reader device isdescribed, which includes various functional modules, such as a timingmodule, a token interface module, a priority-assignor module, and asynchronization module. The device is configured to carry out thevarious operational embodiments of the present invention.

These and other objects, advantages and features will become readilyapparent in view of the following detailed description of the invention.Note that the Summary and Abstract sections may set forth one or more,but not all exemplary embodiments of the present invention ascontemplated by the inventor(s).

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form a partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and to enable a person skilled in the pertinent art to makeand use the invention.

FIG. 1 illustrates an environment where RFID readers communicate witheach other as well as with an exemplary population of RFID tags,according to an embodiment of the present invention.

FIG. 2 shows various Local Area Network (LAN) protocols, includingconventional token passing protocols, mapped to the Open SystemInterconnection (OSI) Reference Model.

FIG. 3A shows an example token passing ring topology.

FIGS. 3B, 4, and 5 illustrate flowcharts depicting various methods ofoperation for efficient tag communication, according to embodiments ofthe present invention.

FIG. 6 shows a token loop with redundancy, according to an embodiment ofthe present invention.

FIG. 7 shows a master-slave reader arrangement, according to anembodiment of the present invention.

FIGS. 8A and 8B show linear and planar arrangements, respectively, forspatially separated readers synchronized in a group, according toembodiments of the present invention.

FIG. 9 shows a reader having multiple functional modules, according toan example embodiment of the present invention.

FIG. 10 shows a time chart having a plurality of reader selectable timeslots, according to an example embodiment of the present invention.

FIG. 11 shows a reader having further exemplary functional modules,according to an embodiment of the present invention.

The present invention will now be described with reference to theaccompanying drawings. In the drawings, like reference numbers indicateidentical or functionally similar elements. Additionally, the left-mostdigit(s) of a reference number identifies the drawing in which thereference number first appears.

DETAILED DESCRIPTION OF THE INVENTION

Introduction

The present invention relates to systems and methods for readernetworks, and for optimizing RFID tag interrogations when a plurality ofreaders desire to communicate with a particular population of tags.According to an embodiment of the present invention, the plurality ofreaders form a network and communicate among themselves. The readernetwork incorporates token passing to determine the timing of readercommunications.

Before discussing details of token passing, an example readerenvironment is described with respect to FIG. 1. FIG. 1 illustrates anenvironment 100 where RFID tag readers 104 communicate with an exemplarypopulation 120 of RFID tags 102. As shown in FIG. 1, the population 120of tags includes seven tags 102 a-102 g. According to embodiments of thepresent invention, a population 120 may include any number of tags 102.

Environment 100 includes a plurality of readers 104, such as readers 104a-104 c. According to embodiments of the present invention, readernetwork 106 may include any number of readers 104, including tens,hundreds, or even more of readers 104. Reader network 106 can bereferred to as a “dense reader” network, and environment 100 can bereferred to as a “dense reader environment,” when a large number ofreaders are operating as members of the network.

In an embodiment, a reader 104 may be requested by an externalapplication to address the population of tags 120. Alternatively, reader104 may have internal logic that initiates communication, or may have atrigger mechanism that an operator of reader 104 a uses to initiatecommunication.

As shown in FIG. 1, readers 104 transmit an interrogation signal 110having a carrier frequency to the population of tags 120. Readers 104operate in one or more of the frequency bands allotted for this type ofRF communication. For example, frequency bands of 902-928 MHz and2400-2483.5 MHz have been defined for certain RFID applications by theFederal Communication Commission (FCC).

Various types of tags 102 may be present in tag population 120 thattransmit one or more response signals 112 to an interrogating reader104, including by alternatively reflecting and absorbing portions ofsignal 110 according to a time-based pattern or frequency. Thistechnique for alternatively absorbing and reflecting signal 110 isreferred to herein as backscatter modulation. Readers 104 receive andobtain data from response signals 112, such as an identification numberof the responding tag 102.

In addition to being capable of communicating with tags 102, readers 104a-104 c communicate among themselves in a reader network, according toembodiments of the present invention. Each of readers 104 a-104 ctransmits reader signals 114 to others of readers 104 a-104 c, andreceives reader signals 114 from others of readers 104 a-104 c.

With multiple readers configured to interrogate tags using the samecommunications medium (air interface in this case), there is apossibility of media contention. Media contention occurs when two ormore readers have an interrogation signal to send to the network at thesame time. Various techniques are used to allow only one reader toaccess the network at a time. This is conventionally done in two mainways: carrier sense multiple access collision detect (CSMA/CD) and tokenpassing.

In networks using CSMA/CD technology, such as Ethernet, when a devicehas data to send, it first “listens” to see if any other device iscurrently using the network. If not, the device starts sending its data.After finishing its transmission, it listens again to see if a collisionoccurred. A collision occurs when two devices send data simultaneously.When a collision happens, each device waits a random length of timebefore resending its data. In most cases, a collision will not occuragain between the two devices. Because of this type of networkcontention, the busier a network becomes, the more collisions occur.This is why the performance of Ethernet systems degrades rapidly as thenumber of devices on a single network increases.

In token-passing networks such as Token Ring networks and FiberDistributed Data Interface (FDDI) networks, a token is passed around thenetwork from device to device. A token is a special form of data signal,usually of short length, that contains some sort of control information.In a networking environment, where multiple devices are configured tocommunicate with the network, possession of the token enables aparticular device to transmit data onto the network. The access methodby which network devices access the physical medium in an orderlyfashion based on possession of the token is known as “token passing”.

When a reader has data to send, it must wait until it has the token andthen sends its data. When the data transmission is complete, the readertemporarily relinquishes its access to the communication medium, i.e.the token is released to the network. The network then sends signals tothe next reader so that next reader may use the communication medium,i.e. the ‘token is passed’ from one reader to another.

One of the advantages of token-passing networks is that they can be madedeterministic, if necessary. In other words, it is possible to calculatethe maximum time that will pass before a device has the opportunity tosend data. This feature makes the token passing protocol very popular inreal-time monitoring operations, such as product tracking in warehousesand process control in factories.

In the 1970s, IBM developed the Token Ring network as a local-areanetwork (LAN) technology based on token passing. As indicated by thenomenclature, the Token Ring network is based on a ring topology. A ringtopology is a network topology that consists of a series of networknodes that are connected to one another by unidirectional transmissionlinks to form a single closed loop. A ring topology can be organized asa star implementing a unidirectional closed-loop star.

The Institute for Electrical and Electronics Engineers (IEEE) laterdeveloped the IEEE 802.5 specification to be very similar to andcompletely compatible with IBM's Token Ring network. IEEE 802.5 is a LANprotocol that specifies an implementation of the physical layer andMedia Access Control (MAC) sublayer of the data link layer using tokenpassing access at 4 to 16 Mega bits per second (Mbps) over shieldedtwisted pair (STP) cabling.

A detailed description of the IEEE 802.5 protocol may be found in “TokenRing/IEEE 802.5”, Chapter 9 of the Internetworking TechnologiesHandbook, copyrighted 1992-2005 to Cisco Systems, which is incorporatedby reference herein in its entirety.

FIG. 2 shows conventional LAN protocols mapped to the Open SystemsInterconnection Reference Model (OSI Reference Model for short). The OSIReference Model is a layered abstract description for communications andcomputer network protocol design, developed as part of the Open SystemsInterconnect initiative. It is also called the OSI seven layers model.LAN protocols function at the lowest two layers of the OSI model, i.e.between the physical layer, and the data link layer.

In IEEE 802 local area networks, and some non-IEEE 802 networks such asFDDI, the data link layer may be split into a Media Access Control (MAC)layer and the IEEE 802.2 Logical Link Control (LLC) layer, as shown inFIG. 2. A more detailed description of the conventional LAN protocolsmay be found in “Introduction to LAN Protocols”, Chapter 2 of theInternetworking Technologies Handbook, copyrighted 1992-2005 to CiscoSystems, which is incorporated by reference herein in its entirety.

It is noted that though the IBM Token Ring and the IEEE 802.5 topologiesexist, embodiments of the present invention differ from the conventionalprotocols.

For example, one significant difference between embodiments of thepresent invention and conventional token passing schemes is that theprimary purpose of inter-device communication according to embodimentsof the present invention is to relay permission to utilize the RF mediumfrom one RFID reader to the next. No actual RF tag interrogation isperformed over the wired links connecting the networked devices. In anembodiment, readers are coupled together in the ring topology by wiredlinks for inter-device communication. These wired links may enablevarious inter-device communication functions, including establishingsynchronization, coordinating RF transmissions, and maintainingsynchronization robustly among multiple readers. In another embodiment,inter-device communication may take place via wireless links operatingat a frequency range different from the tag interrogation RF frequencyrange.

Additionally, in an embodiment, there is no interposing network adapteror protocol stack (as shown in FIG. 2) between the readers.Synchronization signals are passed that serve to directly trigger RFtransmission through air. The readers act upon the synchronizationsignal in order to achieve contention-less communication with lowlatency. This allows a set of readers in close proximity totime-multiplex the RF medium in a very efficient manner.

It is noted that references in the specification to “one embodiment”,“an embodiment”, “an example embodiment”, etc., indicate that theembodiment described may include a particular feature, structure, orcharacteristic, but every embodiment may not necessarily include theparticular feature, structure, or characteristic. Moreover, such phrasesare not necessarily referring to the same embodiment. Further, when aparticular feature, structure, or characteristic is described inconnection with an embodiment, it is submitted that it is within theknowledge of one skilled in the art to affect such feature, structure,or characteristic in connection with other embodiments whether or notexplicitly described.

EXAMPLE READER OPERATIONAL EMBODIMENTS

FIG. 3A shows an example token passing ring topology. A token passingnetwork 350 includes readers 352 a-352 f arranged in a ring with links354 a-354 f coupling readers in the ring. Each of readers 352 a-352 faccesses a communication medium when in possession of a token, and thenpasses the token on to the next reader via links 354 a-354 f. Forinstance, after accessing the communications medium (e.g. byinterrogating a tag), reader 352 a passes the token to reader 352 b vialink 354 a.

FIG. 3B shows a flowchart 300 providing steps for example communicationsin a reader network, according to the present invention. Flowchart 300relates to an example token passing protocol. The steps of flowchart 300can be performed by embodiments of readers described herein. Otherstructural and operational embodiments will be apparent to personsskilled in the relevant art(s) based on the following discussion relatedto flowchart 300. The steps shown in FIG. 3B do not necessarily have tooccur in the order shown.

Flowchart 300 begins with step 302. In step 302, a reader enters thetoken passing mode. For example, reader 352 a of FIG. 3A is configuredto operate in a token passing mode.

In step 304, the reader waits for a predetermined time interval. Asdescribed in further detail below, in an embodiment, readers in a tokenpassing network do not access the communication medium withoutcoordinating among themselves in order to decrease the likelihood ofsimultaneous RF transmission. Thus, the readers do not sendinterrogation signal as soon as they enter the token passing mode, butwait till they possess a token.

In step 306, the reader possesses the token. As elaborated later, atoken passing network might have more than one circulating tokens.However, the token that enables a particular reader to access the RFmedium is called a “primary token”. Unless otherwise mentionedspecifically, the terms “token” and “primary token” are usedinterchangeably herein.

A reader may receive the token from another reader in the token passingnetwork, or the reader may be configured to generate a token, ifrequired. Possession of the token enables the reader to access thecommunication medium for tag interrogation. However, the reader mightnot necessarily perform tag interrogation each time it receives a token.

In step 308, the reader determines if it needs to access thecommunication medium for tag interrogation. If the reader needs toaccess the communication medium, operation proceeds to step 310. If not,operation proceeds to step 312.

For example, the reader may determine it needs to access thecommunication medium, if an operator of the reader initiates a read of atag, the reader receives a remote command to initiate a read of a tagpopulation, or other mechanism triggers the interrogation. In step 310,the reader accesses the communication medium to interrogate at least onetag.

In step 312, the reader retains the token for a variable period of time.In an embodiment, it is not mandatory that a reader passes the tokenimmediately after it has finished its interrogation. Even if the readerdoes not need to access the communication medium, as determined in step308, the reader may choose to retain the token for some time, ifdictated by the particular situation or network configuration. Theretention time can vary from zero to a pre-set maximum interval, forexample.

In step 314, the reader passes the token to another reader. For example,in FIG. 3A, reader 352 a passes the token to reader 352 b. However,reader 352 a may be configured to pass the token to a plurality ofreaders.

Steps 306-314 may subsequently be repeated by the next reader to receivethe token. For example, reader 352 b repeats the steps 306-314.

Within a token passing network, typically only one instance of a primarytoken is circulated. However, while possession of the primary tokenenables a reader to access the communication medium, there may be one ormore secondary tokens. The role of a secondary token is typically ofadvisory nature. For example, a secondary token may alert a reader thatit is next in line to receive the primary token.

Flowchart 400 in FIG. 4 further elaborates the concept of a readerwaiting for a predetermined time interval before tag interrogation, asdescribed in flowchart 300 with respect to step 312. The steps shown inFIG. 4 do not necessarily have to occur in the order shown.

As shown in FIG. 4, in step 402, a reader waits a predetermined timeinterval. Upon entering a token passing mode, the reader listens forincoming signals for a variable period of time. The variable period oftime can be predetermined to be greater than the length of the ringcycle time, i.e. the time required for a token to travel around theentire token passing network at least once through each of the pluralityof RFID readers in the network.

Step 403 is optional, depending on the particular system configuration.In a system having redundancy, as described below with reference to FIG.6, a reader may receive a secondary token in step 403. Typically, thesecondary token is received before the ring cycle time expires.

In step 404, it is checked whether the reader receives a primary tokenwithin the pre-determined time interval. If the reader does not receivea primary token within the predetermined time interval, operationproceeds to step 406. If the reader does receive the primary token,operation proceeds to step 408.

In step 406, the reader generates a primary token, and operationproceeds to step 408.

In step 408, the reader accesses the communication medium for taginterrogation.

In step 414, the reader passes the token to another reader. Note that areader may be configured to pass the token to a plurality of readers.

Readers in a token passing network may be arranged in any topology,including tree topology, star topology, and ring topology. FIG. 6 showsan example token loop or token ring network 600 with provision forredundancy. Each of the readers in network 600 may interrogate tagswirelessly using RF communication through air. At the same time, theremay be separate wired hardware links coupling the readers together.Inter-reader signaling or token passing happens through the real timewired links.

As shown in FIG. 6, network 600 includes readers 602 a-602 f. There maybe any number of individual readers included in the network. Links 604a-604 f couple corresponding adjacent readers 602 in network 600, andrepresent wired links for transferring a primary token. Links 606 a-606f represent wired links for transferring a secondary token.

As described before, in an embodiment, the reader that receives theprimary token is allowed to access the RF communications medium, whilethe reader that receives the secondary token becomes aware that it willpossess the primary token next. A secondary token may help in efficientpower management and faster operation in a network. For example, if areader goes into power-down mode to save power after finishing its taginterrogation during the previous cycle, receiving a secondary token maytrigger powering-up of the device, so that it can perform taginterrogation in the current cycle without much latency after receivingthe primary token.

A secondary token may also serve as a tool for ensuring robustness in aredundant system, safeguarding system integrity. A redundant systemprevents a single point of failure from shutting down the operation ofthe entire system. In the event of a primary token link failure, thenetwork will switch to backup option, such as using a secondary tokenlink to keep the system interruption at a minimal level. One or morereaders in the network are configured to generate and transfer primaryand secondary token signals in a redundant token ring.

In the example embodiment shown in FIG. 6, the primary token is passedfrom one reader to its adjacent reader, for example, from reader 602 ato reader 602 b via link 604 a, while the secondary token is sent to thereader yet one position further in the ring, for example, from reader602 a to reader 602 c via link 606 b. As shown in FIG. 6, each primarytoken link 604 couples adjacent readers, while each secondary token link606 couples a reader to the second reader coupled in series in theforward direction clockwise in network 600.

If the ring's integrity is intact, reader 602 c receives the primarytoken from reader 602 b within a predetermined time period, as describedin flowchart 400. However, if the ring is disrupted, for example, iflink 604 b is broken (due to power failure of reader 602 b, or damageincurred by the physical hardware connecting readers 602 b and reader602 c), reader 602 c, already possessing the secondary token from reader602 a, may reasonably assume that after a predetermined time, it shouldhave received the primary token, and therefore, reader 602 c is entitledto transmit tag interrogation signal. As shown in flowchart 400, reader602 c thus generates a primary token, and accesses the communicationmedium.

The time interval between receiving the secondary token and the primarytoken is referred to as the “gap time-out”.

Another time-out pertinent to the token passing network operation isreferred to as the “loop time-out”. When a reader does not receive anyform of token (neither primary nor secondary) for a sufficiently longperiod of time, the reader can reasonably assume that it should havereceived a token, and is entitled to generate a primary token as well.This time interval is known as “loop time-out” and can be predetermined,generated based on the ring network characteristics, or otherwisedetermined.

In order to ensure that each reader in the token passing network gets tointerrogate corresponding tags at least once within a reasonable amountof time (which may vary according to system requirements), the notion oftime-budget can be incorporated as an operational feature in eachreader. Time-budgeting is a process that determines how long aparticular reader can retain a primary token in a token passing networkto maintain the overall efficiency of the network operation.

Readers within a ring can be assigned different levels of priority. If areader has a large number of tags in its read zone, the reader maydesire to retain the token for a longer time. Therefore, such a readermay be assigned a higher priority. Similarly, a reader deemed to have aless demanding environment, such as a reader having a lesser number oftags to read, or having a greater allowable time interval betweensubsequent reads, may be assigned a lower priority.

A reader with a lower priority may be configured to relinquish the tokena predetermined number of times (‘n’ times) without performing an RFinterrogation. However, the lower priority reader may be required tointerrogate tags at least once when it receives the token for the(n+1)-th time.

Readers with different levels of priority may adjust the token retentiontime according to their particular requirements. The priority levels maybe fixed or may change as the reader's environment changes with time.

In an embodiment, a reader with the highest level of priority retainsthe token for a “maximum hold-time”, which is predetermined. Setting amaximum hold-time helps to prevent missed read opportunities by thelower priority readers caused by higher priority readers monopolizingthe air interface. The maximum hold-time can be set to different preset(or determined ‘on the fly’) values as the reader environment changes.

One of the ways a reader environment may change is if a second RF systemconsisting of a different set of networked readers and the same or adifferent set of tags with respect to the present RF system, needs toshare the same communication medium. For example, a second RF system maybe temporarily brought in for a random inventory checking, while thefirst RF system has routine inventory checking performed at a regularinterval. FIG. 5 depicts a flowchart 500 that describes the operation ofa reader in the first token passing network when there is a second RFsystem introduced in the environment. The steps of flowchart 500 areexemplary, and need not happen in any particular order. There may beadditional steps not shown in FIG. 5, which may be apparent to anordinarily skilled artisan. Flowchart 500 illustrates how it is possiblefor a reader to create gaps in RF medium utilization.

In step 502 of flowchart 500, a reader checks its environment todetermine the need for the second RF system to access the communicationsmedium.

In step 504, if it is determined that the second RF system needs toaccess the communications medium, then the reader proceeds to step 508.

In step 508, the reader calculates a token retention time. Thecalculation is based on the requirement of the second RF system. Forexample, the calculation may take into account one or more of thefollowing: the number of readers in the second RF system, the number oftags in the second RF system, the frequency of interrogation, thepriority level of the second RF system, etc. As described further below,a timing module in a reader is capable of calculating the tokenretention time.

In step 510, the reader retains the token for the calculated retentiontime. For example, after the reader has completed its interrogations, itcan delay the next reader in its loop by not sending the token for thecalculated retention time. This step creates an opportunity for thesecond RF system to utilize the RF medium and RF spectrum. However, theretention time should be less than the maximum hold-time aspredetermined by the system.

If it is determined in step 504 that the second RF system does not needto access the communications medium, then the reader proceeds to step506. In step 506, the reader retains the token for a pre-set time.

In step 512, the reader passes the token to another reader. Thus, thetoken starts circulating again within the first RF system after allowingthe second RF system to perform at least one interrogation.

EXAMPLE SPATIAL ARRANGEMENT OF THE TOKEN PASSING READERS

The token ring described above may be extended to include a master-slaveconfiguration, as depicted in FIG. 7. FIG. 7 shows a network 700 havinga master-slave configuration, comprising master readers 702 a-702 d;slave readers 703 a-703 d, 704 a-704 d, and 705 a-705 d; and links 720,730, 740, 750, 721 a-721 d, 722 a-722 d, and 723 a-723 d. Link 730couples master readers 702 a and 702 b, link 740 couples master readers702 b and 702 c, link 750 couples master readers 702 c and 702 d, andlink 720 couples master readers 702 d and 702 a. Links 721 a, 722 a, and723 a couple slave readers 703 a, 704 a, and 705 a, respectively, withtheir corresponding master reader 702 a. Links 721 b, 722 b, and 723 bcouple slave readers 703 b, 704 b, and 705 b, respectively, with theircorresponding master reader 702 b. Links 721 c, 722 c, and 723 c coupleslave readers 703 c, 704 c, and 705 c, respectively, with theircorresponding master reader 702 c. Links 721 d, 722 d, and 723 d coupleslave readers 703 d, 704 d, and 705 d, respectively, with theircorresponding master reader 702 d. In such a master-slave configuration,each reader in a ring topology can emit its token passing signal to aplurality of readers, instead of just the next reader in line. Only oneof these readers need to be part of a ring topology. In the examplearrangement of FIG. 7, readers 702 a-702 d are part of a token passingloop, with links 720, 730, 740, and 750 being the token transfer linksconnecting them in a token ring. One or more of readers 702 a-702 d havea number of slave readers associated with them. For example, readers 703a, 704 a, and 705 a are configured to act as slaves of the master reader702 a. It is noted that though in FIG. 7, each of master readers 702a-702 d are shown to have an equal number of slave readers associatedwith them, a master reader can have any number of slave readers.

A slave reader receives a token from its corresponding master reader viaa corresponding link, and sends the token back to the master reader. Themaster reader generates a slave token to be transferred to itscorresponding slave readers. For example, slave readers 703 a, 704 b,and 705 a engage in receiving and sending slave tokens with masterreader 702 a only, via links 721 a, 722 a, and 723 a, respectively. Aslave reader is typically not enabled to pass the slave token to anotherslave reader, or to another master reader in the loop. However, in anembodiment, a slave reader can be a secondary master reader for its ownset of slave readers.

It is noted that the previously described methods of redundancy,time-budgeting, etc. are applicable to the master-slave configuration.

A token passing network may include one or more local networks,spatially separated from one another, where each of the local networksincludes one or more readers that circulate a local token restrictedwithin that local network. One or more of the readers in a local networkcirculate the primary local token associated with that local network intemporal synchronization with corresponding readers in another localnetwork. Synchronized readers perform tag interrogation simultaneously.

Members belonging to different local networks, but the samesynchronization group, are spatially separated to mitigate interference.Spatial separation can manifest in a one-dimensional linear arrangement800, as shown in FIG. 8A, or in a two-dimensional planar arrangement850, as shown in FIG. 8B.

For example, in FIG. 8A, a local network 801 a includes readers 802 a,804 a, 806 a, and 808 a, a local network 801 b includes readers 802 b,804 b, 806 b, and 808 b, and a local network 801 c includes readers 802c, 804 c, 806 c, and 808 c. Though belonging to different localnetworks, readers 802 a, 802 b, and 802 c are synchronized as a group.Thus, readers 802 a, 802 b, and 802 c will each have possession of arespective local token at the same time as each other. Furthermore, theyeach pass their respective local token to their respective next readersimultaneously (i.e. pass local token to readers 804 a, 804 b, and 804c, respectively). This holds true for subsequent readers in the localnetworks.

In a two-dimensional arrangement 850, shown in FIG. 8B, readers 810 a,810 b, 810 c, 810 d, 810 e, and 810 f are synchronized as a group, evenif they are spatially apart, and belonging to different local networks.

Synchronized groups of readers may be arranged in any otherconfiguration, including a three-dimensional stacked configuration.

EXAMPLE APPARATUS EMBODIMENTS

Embodiments of the present invention improve upon existing inter-readersignaling approaches, to better optimize usage of the RF communicationsmedium for improved efficiency of reader to tag communications.

For example, FIG. 9 shows a reader 900 according to an embodiment of thepresent invention. Reader 900 has at least one antenna 908, and acontroller unit 916 that includes various functional modules, includinga tag communications module 910 coupled to the at least one antenna 908,a token interface module 920, and a timing module 902 coupled to tagcommunications module 910.

Token interface module 920 is configured to receive a token signal fromanother reader through link 940, and transmit a token signal to anotherreader through link 930. In an embodiment, links 930 and 940 may bewired links capable of transmitting signals with minimal latency.

Timing module 902 monitors communications performed by tagcommunications module 910. For example, timing module 902 determines howmuch time elapses during a particular tag interrogation by tagcommunications module 910. Furthermore, timing module 902 determines howmany readers are present in the local environment and how many frequencychannels are available. Timing module 902 calculates a desired tokenretention time using the determined information, as described above withreference to flowchart 500 in FIG. 5.

Timing module 902 provides a timing signal 904 to tag communicationsmodule 910 to indicate to tag communications module 910 how long after atag interrogation the token is passed to another reader. Timing module902 can be implemented in hardware, software, firmware, and anycombination thereof.

It is typically desired to minimize the likelihood of multiple instancesof primary token signals from existing within a token ring. Multipleprimary tokens result in multiple readers occupying the communicationsmedium simultaneously, defeating the intended goal of the token passingarchitecture. In an embodiment, when multiple readers can select anoccupied frequency channel, upon entering the token passing mode, thereaders listens for incoming signal for a random period of time. Thisrandom period may be a multiple of an amount of time that is greaterthan the interval required for a token to travel the entire circuit ofreader devices. This creates, in effect, “time slots”. For example, FIG.10 shows a time slot chart 1000 having a plurality of reader selectabletime slots 1002 a-1002 j. In FIG. 10, for illustrative purposes, tentime slots 1002 a-1002 j are shown, but in embodiments, any number oftime slots may be present.

In the present embodiment, when multiple readers contend for a frequencychannel that is already in use, each reader selects one of time slots1002 a-1002 j, in a random or other fashion. The reader that selects thefirst slot secures the frequency channel first, and can communicate onthe frequency channel first. The reader that selects the earliest slotcreates a primary token. The existence of a primary token prevents otherreaders securing later slots from creating another instance of primarytoken. Any reader that has selected a later time slot can continue towait till they receive a token indicating that the frequency channel isclear. In case a primary token signal does not reach a reader after acertain predetermined period of time (e. g. because of link failure orother reasons), the reader may generate a primary token at the beginningof the time slot that it has chosen.

FIG. 11 shows a reader 1100 including at least one antenna 908, a tagcommunications module 910, a timing module 1102, a token interfacemodule 920, a priority assignor module 1150, and a synchronizationmodule 1160. Token interface module 920 includes token input module1140, and token output module 1130.

Timing module 1102 includes a time slot selector 1101 to select a timeslot for reader 1100, according to an embodiment of the presentinvention. Time slot selector 1101 is coupled to tag communicationsmodule 910, and controls which time slot tag communications module 910initiates a tag interrogation (or other communication). As shown in FIG.11, time slot selector 1101 can have a random number generator 1104.Random number generator 1104 may be used to select a time slot in arandom manner. Time slot selector 1101 may also have a timer (not shown)for determining when a selected time slot has arrived. Time slotselector 1101 and random number generator 1104 (when present) can beimplemented in hardware, software, firmware, and any combinationthereof.

As shown in FIG. 11, reader 1100 includes a priority-assignor module1150, and a synchronization module 1160, both coupled to the timingmodule 1102. Synchronization module 1160 is also coupled to tokeninterface module 920.

As described before, priority-assignor module 1150 assigns a valuerepresenting the priority level to reader 1100 in the network. The tokenretention time (i.e. the length of time a reader retains a token beforetransferring it to the next reader) is calculated depending on thepriority level of the reader.

Synchronization module 1160 sends a synchronization signal 1162 totiming module 1102. As shown in FIGS. 8A and 8B, spatially separatedreaders can synchronously interrogate tags. If readers belonging to thesame synchronous group (e. g. 802 a, 802 b, 802 c in FIG. 8A) receivethe primary and secondary tokens from their respective local networks atdifferent instances of time because of local loop irregularities, thensynchronization module 1160 enables timing module 1102 to adjust thetoken retention time, so that a tag interrogation signal is sent outsynchronously with other members of the group.

It is noted that in some embodiments, synchronization module 1160 maynot be needed, because inexactness in local loops can actually behelpful in ‘settling’ the network to a synchronous token state. Readerstypically listen for tokens during the entire interval of each slot, andmay generate a token at the beginning of each pre-selected slot.Therefore the network is inherently resilient to non-synchronouspowering-up to some extent. However, if the local loop inexactness isirregularly high (for example, in case of multiple links failure), thenthe synchronization module 1160 may be necessary to act to correctpotential asynchronous operation that may lead to RF interference.

Token input module 1140 receives one or more token inputs 940 from otherreaders in the network. Typically, each reader resets its token inputs940 when it is about to send out a token output signal 930, generated bythe token output module 1130, to another reader. Therefore the reader‘forgets’ the occurrence of spurious tokens, if any. If a redundanttoken, such as a secondary token, travels around a loop, it shouldtravel synchronously, which is not a stable condition, due to thenon-static nature of RFID interrogation. Synchronization module 1160 isconfigured to remove multiple and asynchronously traveling tokens toensure interference-free network operation.

CONCLUSION

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. It will be apparent to persons skilledin the relevant art that various changes in form and detail can be madetherein without departing from the spirit and scope of the invention.Thus, the breadth and scope of the present invention should not belimited by any of the above-described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

1. A method in a first reader for time multiplexing access to acommunication medium in a first radio frequency identification (RFID)communications system that includes a plurality of RFID readers, whereinthe method comprises the steps of: (a) entering into a token passingmode; (b) waiting for a predetermined time interval before accessing thecommunication medium; (c) possessing a token, possession of whichpermits the reader to access the communication medium for taginterrogation; (d) determining whether to access the communicationmedium for tag interrogation; (e) retaining the token for a period oftime; and (f) passing the token to a second reader, wherein step (e)further comprises determining a time limit, wherein the first reader isenabled to possess the token for a time interval less than or equal tothe determined time limit, after which the first reader transfers thetoken to a second reader in the network, wherein step (e) furthercomprises: analyzing an environment to determine whether a second RFIDcommunications system needs to access the communications medium for taginterrogation; calculating a time interval greater than the timerequired by the second communications system to perform at least one taginterrogation, if it is determined that the second communications systemneeds to access the communications medium; and prior to performing step(f), adjusting a token retention time to be equal to the time intervalcalculated greater than the time required by the second communicationssystem to perform the at least one tag interrogation.
 2. The method ofclaim 1, further comprising: (g) accessing the communication medium if aneed to interrogate tags is determined by the reader before performingstep (f).
 3. The method of claim 1, wherein in step (b), thepredetermined interval of time is a multiple of a time interval greaterthan a cycle time, wherein the cycle time is a minimum time required fora token to be transferred at least once to each of the plurality of RFIDreaders in the network.
 4. A radio frequency identification (RFID)communications system, comprising: a plurality of RFID readers that forma token passing network, wherein at least two of the readers areconfigured to receive and transfer a token for time multiplexing accessto a communication medium to perform at least one RF tag interrogation;and a primary reader that generates a primary token, possession of whichenables a reader to access the communication medium for taginterrogation, and, wherein the primary reader transfers the primarytoken to another reader in the token passing network, wherein theprimary reader randomly selects a time slot during which the primaryreader is enabled to access the communication medium for taginterrogation.
 5. The system of claim 4, further comprising: apopulation of RFID tags, wherein each tag of the population is withinthe interrogation range of at least one of the RFID readers.
 6. Thesystem of claim 4 wherein readers of the plurality of readers other thanthe primary reader occupy time slots later than the time slot selectedby the primary reader, wherein a reader occupying a later time slot isinhibited from generating a primary token as long as a previouslygenerated primary token exists in the token passing network,
 7. Thesystem of claim 4, wherein a reader is allowed to generate a token onlyat the beginning of a pre-selected time slot.
 8. The system of claim 4,wherein a reader of the plurality of RFID readers comprises a secondarytoken generator that generates a secondary token, possession of whichalerts a reader that it will possess the primary token next.
 9. Thesystem of claim 8, wherein the secondary token is transferred from afirst reader to a third reader, bypassing at least a second readercoupled in between the first reader and the third reader.
 10. The systemof claim 8, wherein a reader possessing the secondary token is allowedto access the communication medium for tag interrogation if apredetermined interval of time has passed, and the reader possessing thesecondary token has not currently received the primary token.
 11. Thesystem of claim 4, wherein a reader of the plurality of readers is amaster reader configured to generate a slave token to transfer to atleast one slave reader coupled to the master reader.
 12. The system ofclaim 11, further comprising one or more slave readers configured toreceive the slave token from the corresponding master reader, and sendthe slave token back to the master reader.
 13. The system of claim 4,wherein the network comprises: one or more local networks, spatiallyseparated from one another, wherein each of the local networks comprisesone or more readers that circulate a local token, and wherein one ormore of the readers in a first local network circulate a first localtoken in temporal synchronization with readers in a second local networkthat circulate a second local token,
 14. The system of claim 13, whereintemporally synchronized readers are spatially arranged in a onedimensional linear arrangement, two dimensional planar arrangement or athree dimensional stacked arrangement.
 15. The system of claim 4,wherein each of the readers is assigned a value corresponding to apriority level in the network.
 16. The system of claim 15, wherein areader with an assigned value indicating a lower priority is configuredto pass the token for a pre-determined fixed number of ‘n’ times withoutperforming an RF interrogation, and wherein the reader is required tointerrogate tags at least once when it receives the token for the(n+1)th time.
 17. A radio frequency identification (RFID) reader,comprising: at least one antenna; a tag communications module coupled tothe at least one antenna and configured to interrogate tags; a tokeninterface module configured to generate and transfer a token to anotherreader within a token passing reader network; and a timing modulecoupled to the tag communications module and the token interface module,wherein the timing module calculates a desired variable time intervalfor interrogating tags.
 18. The RFID reader of claim 17, wherein thereader further comprises: a priority-assignor module, that assigns avalue indicating the relative priority level of the reader in thenetwork; a synchronization module coupled to the timing module, thesynchronization module enabling the reader to synchronize taginterrogation with other readers that are spatially separated.